A complex bearing assembly or mechanism, which includes a hydrodynamic bearing for the radial support and a magnetic bearing for the axial support, has widely been employed as one component of a spindle motor for use in a magnetic memory device such as hard disc drive and a laser scanner for attaining a high-speed and high-precision rotation of the motor.
In this conventional complex bearing assembly, in particular a frictional wear at the radial dynamic bearing has been considered as a problematic matter that should be solved. Specifically, at the very beginning of the rotation of the motor in which the rotational speed of the motor has not been increased enough, the hydrodynamic pressure generated in the radial bearing is still small. Disadvantageously, this results in a continuous and/or discontinuous frictional contact in between a support member and a rotatably member supported for rotation, e.g., a fixed sleeve and a shaft inserted in the sleeve. Then, when the rotational speed of the motor is increased to a certain extent and, thereby, a rotational axis of the rotating member is stabled, the frictional contact between the support and rotatable members will come to an end.
As is known in the art, the frictional contact results in a frictional wear that decreases a durability of the spindle motor. The frictional wear depends upon the transitional rotational speed at which the frictional contact between the shaft and the sleeve comes to an end. That is, the frictional wear increases in proportion to the transitional rotational speed. This in turn means that, in order to attain a highly reliable bearing assembly and spindle motor having such bearing assembly, the transitional rotational speed is required to be minimized.
In the meantime, in the complex bearing assembly in which the shaft is extended vertically, a load or gravity of a rotational portion or rotor of the bearing assembly acts on the axial bearing and a centrifugal force caused by an unbalanced force of the rotor acts on the radial bearing. Also, no static force acts in a certain direction on the radial bearing, which in turn means that a portion where the radial bearing receives the centrifugal force moves with the rotation of the rotor.
Therefore, when the rotor rotates at a relatively low speed, the centrifugal force due to the unbalanced force of the rotor is greater than the hydrodynamic force derived in the radial bearing, which allows the sleeve to contact with the shaft. The contact portion between the shaft and sleeve moves around the shaft continuously. If the contact could be retained until a relatively high rotational speed of rotor, an increased contact force or energy consumption between the contacting rotor and shaft will provide a significant wear or damage to both rotor and shaft.
A most effective way to solve those problems is to increase the hydrodynamic force at the lower rotational speed of the rotor. In general, the hydrodynamic force changes according to diameter, length, and eccentricity of the bearing members and clearance between opposing sleeve and shaft. Specifically, the hydrodynamic force increases with the increase of diameter and length and the with the decrease of the clearance. The increase of the diameter and length results in an increase of the size of the motor and a system incorporating the motor, which fails to meet the current tendency of a compactness of the device. Also, a reduction in clearance requires a significant precision for the manufacturing of the shaft and sleeve. Further, the eccentricity, which is determined by the unbalanced mass, can not be controlled in practice.
On the other hand, the increase of diameter, length, or the decrease of clearance may result in a great increase of the rotational friction or resistance, which gives rise to an increase of power consumption by the motor that incorporates such bearing. In addition, the conventional hydrodynamic bearing, which changes its performance with the temperature at use, decreases its threshold rotational number for an instability such as half frequency whirl.